Method of determining optimal shapes for stator blades

ABSTRACT

In an effort to prevent vibration-induced cracking of stator blades of the type that are secured at their radially inner and outer ends a complete stator assembly is made up and its vibration characteristics are investigated. The normal blades are then replaced by blades which are curved to the shape assumed by the normal blades in the predominant mode of vibration found by the investigation. This technique has been found to reduce vibration substantially.

United States Patent 11 1 Pask et a1.

[ 51 July 17,1973

[ METHOD OF DETERMINING OPTIMAL SHAPES FOR STATOR BLADES [75] Inventors:George Pask, Stanton-By-Bridge;

Norman Robert Robinson, Duffield, both of England [73] Assignee:Secretary of State for Defence,

London, England [22] Filed: Apr. 12, 1972 [211 Appl. No.: 243,223

Related US. Application Data [62] Division of Ser. No. 47,514, June 18,1970,

abandoned.

[52] US. Cl. 29/1563 B, 29/l56.8 CF, 29/404,

415/119 [51] Int. Cl B23p 15/02, B23q 17/00 [58] Field of Search....415/119; 29/1568 CF, 156.8 R, 156.8 B, 404

[56] References Cited UNITED STATES PATENTS 3,429,023 2/1969 Blythe etal 29/404 2,029,813 2/1936 DeMey 415/216 871,292 11/1907 Parsons et al415/119 UX 1,062,258 5/1913 Schlotter 415/1 19 UX 1,843,786 2/1932Robinson 415/119 3,572,962 3/1971 Embleton et a1. 415/119 FOREIGNPATENTS OR APPLICATIONS 441,269 1/1936 Great Britain 415/119 226,2037/1925 Great Britain 630,277 10/1949 Great Britain 712,589 7/1954 GreatBritain 766,812 1/1957 Great Britain 415/1 19 Primary Examiner-CharlesW. Lanham Assistant ExaminerD. G. Reiley, 111 Attorney-Cushman, Darby &Cushman [57] ABSTRACT In an effort to prevent vibration-induced crackingof stator blades of the type that are secured at their radially innerand outer ends a complete stator assembly is made up and its vibrationcharacteristics are investigated. The normal blades are then replaced byblades which are curved to the shape assumed by the normal blades in thepredominant mode of vibration found by the investigation. This techniquehas been found to reduce vibration substantially.

4 Claims, 5 Drawing Figures METHOD OF DETERMINING OPTIMAL SHAPES FORSTATOR BLADES This is a division of application Ser. No. 47,514, filedJune 18, 1970, now abandoned.

This disclosure relates to stator blades and assemblies thereof for usein fluid flow machines and is'particularly but not exclusively concernedwith stator blades and assemblies thereof for use in an axial flowcompressor of a gas turbine engine.

In this specification the term stator blade-is to be understood toinclude non-rotatable blades such as nozzle guide vanes, inlet guidevanes and the like, and streamlined struts such as are used to supportin spaced relationship the walls of an annular fluid flow duct.

In use, stator blades are subjected to vibration induced, for example,by rotating parts of the fluid flow machine. This vibration can causefatigue and lead eventually to cracking of the blades. It is an objectof the present invention to alleviate this problem.

According to one aspect of the present disclosure a stator bladecomprises radially inner and outer portions adapted to be secured, to,orform part of, the radially inner and outer walls respectively of anannular flow duct in a fluid flow machine and an intermediate por tionconnecting the radially inner and outer portions, the intermediateportion being curved to a shape similar to the shape which would beassumed by a straight, but otherwise substantially identical, blade inuse when vibrating in its predominant mode of vibration.

The amplitude of the curve of the intermediate portion is preferablygreater than the greatest amplitude of the curve which would be assumedby the straight, but otherwise substantially identical, blade.

The intermediate portion is preferably also shaped to exert aerodynamicforces on a fluid flowing through the fluid flow machine.

The disclosure also comprises an axial flow compressor provided with anannular array of stator blades in accordance with any of the precedingstatements of invention, and a gas turbine engine provided with such acompressor.

According to a further aspect of the present disclo-. sure, there isprovided a method of manufacturing an assembly of stator blades, each ofwhich is secured at its radially inner and outer portions, comprisingthe steps of producing an annular array of stator blades each having anintermediate portion which is substantially straight in a radial sense,subjecting the array to operating conditions substantially identical toits desired operating conditions, determining the predominant mode ofvibration of the straight stator blades under said operating conditionsand replacing the straight stator blades with stator blades each havingan intermediate portion which is curved to a shape similar to the shapeassumed by the straight stator blades while vibrating in theirpredominant mode of vibration but which is otherwise substantiallyidentical to the intermediate portion of the straight stator blades.

The disclosure will now be described, merely by way of example, withreference to drawings accompanying the specification in which:

FIG. I is a diagrammatic part-sectional view of a gas turbine engineprovided with a stator assembly in accordance with the presentinvention;

FIG. 2 is a section on the line 2-2 of FIG. 1;

FIG. 3 is an enlarged view of one of the stator blades of FIG. 2; and

FIGS. 4 and 5 are views of alternative versions of the stator blade ofFIG. 3.

In FIG. 1 there is shown an aircraft gas turbine engine 10 comprising,in flow series, an air inlet 12, an axial flow compressor 14, combustionequipment 16, an axial flow turbine 18 which is drivingly connected tothe compressor 14 by a shaft (not shown), a jet pipe 20 and a propulsionnozzle 22. The compressor 14 comprises alternate stages of equiangularlyspaced apart rotor blades 24 and equiangularly spaced apart statorblades 26.

The stator blades 26 each have radially inner and outer portionsconstituted by radially inner and outer shroud members 30 and 32respectively, joined together by intermediate portion consitituted by anaerofoil-shaped working portion 28. The shroud members 30 of adjacentblades 26 are joined together by welding or other suitable means to forma shroud ring which constitutes part of the radially inner wall of theworking duct of the compressor 14, while the shroud members 32 aresimilarly joined to form part of the radially outer wall of the workingduct (see FIG. 2). Both ends of each stator blade 26 are thuseffectively secured to fixed structure. It will be appreciated, however,that other methods of securing the stator blades 26 may be employed: forexample the opposite ends of the working portion 28 may be bolted orwelded to, or located in slots in, the radially inner and outer walls ofthe working duct of the compressor 14.

In FIG. 3 there is shown an enlarged view of one of the stator blades26, and it can be seen that the working portion 28 is curved to a shape(much exaggerated for clarity) substantially similar'to the shape whichwould be assumed by a straight, but otherwise substantially identical,stator blade when vibrating in its first flap mode. In practice, thegreatest amplitude of the curve assumed by the straight blade whenvibrating is such that the curvature is not perceptible to the eye,while the curvature of the working portion 28 is greater than this andis immediately perceptible to the eye.

Since the stator blades 26 are designed to exert aerodynamic forces onthe air flowing through the compressor 14, the side 34 is subjected, inuse, to a higher pressure than the side 36: The working portion 28 maybe curved so that the high pressure side 34 is made concave as a resultof the curve. However, in some applications it may be more advantageousto make the high pressure side 34 convex as a result of the curve, thebetter arrangement for a particular application being determined byexperiment.

The stator blades of FIG. 4 and 5 are basically similar to the statorblade 26, and similar parts will therefore be given the same referencenumerals with the suffix a (FIG. 4) or b (FIG. 5). The working portion28a is curved to a shape substantially similar to the shape which wouldbe assumed by the straight stator blade when vibrating in its secondflap mode, while the working portion 28b is curved to a shapesubstantially similar to the shape which would be assumed by straightstator blade when vibrating in its third flap mode. In both cases, theamplitude of the curve of the working portions 28a, 28b is greater thanthe greatest amplitude of the respective corresponding curve assumed bythe straight blade.

The stator blade assembly of FIG. 2 is manufactured by designing a setof stator blades (not shown) for a desired application in accordancewith known aerodynamic principles, the working portions of the bladesbeing substantially straight in a radial sense. The straight blades arethen assembled to form an annular array of blades similar to that shownin FIG. 2 and subjected to operating conditions substantially identicalto the operating conditions of the desired application. In practice, theannular array of straight blades would be built into the compressor 14which would then be run, either separate from or in the engine 10, underthe normal operating conditions of the engine 10.

The vibration characteristics of the straight blades are theninvestigated while the compressor 14 is running, for example by means ofstrain gauges secured to the blades or by stroboscopic techniques, todetermine the predominant mode of vibration of the straight blades. Inthe example shown in FIG. 2 it has been assumed by the predominant modeof vibration of the straight blades was the first flap mode, althoughthe shape assumed by the straight blades when vibrating in their secondand third flap modes is shown in FIGS. 4 and 5. It will be apprecitaedthat it is possible for the straight blades to vibrate in more than onemode.

The straight blades are then replaced by blades each having a workingportion which is precurved to the shape assumed by the straight bladeswhen vibrating in their predominant mode of vibration but which isotherwise substantially identical to the working portion of the straightblades, i.e. the blades 26.

We have found that the curvature of the working portions 28, 28a or 28bsubstantially reduces the amount of vibration in the respective mode towhich the blades are subjected in use. This is due to the fact that, inorder to vibrate the blades 28, 28a 28b must take up positions on eitherside of their mean or rest positionwhich require a greater change ofblade length than is required by a straight blade when vibrating withthe same amplitude.

This imposes a greater degree of constraint on the curved blades whichhas the effect of increasing the natural frequency of the predominantmode of vibration of the blades of a frequency above the normalexcitation frequencies present when the engine is operating under itsnormal operating conditions.

The present disclosure thus reduces blade fatigue and leads tosignificantly increased blade life.

It will be appreciated that the disclosure is applicable to many typesof stator blades which are secured at each end, for'example nozzle guidevanes, inlet guide vanes, and streamlined struts such as are used tosupport in spaced relationship the walls of an annular fluid flow duct.

We claim:

1. A method of manufacturing an assembly of stator blades having meanssecuring each blade at its radially inner and outer portion, said methodcomprising the following steps:

a. producing an annular array of stator blades, each blade having anintermediate portion which is substantially straight in a radial sense,

b. subjecting said array of stator blades to operating conditionssubstantially identical to the desired operating conditions of saidstator blades,

c. determining the predominate mode of vibration of each of said statorblades under said operating conditions, and

d. replacing each of said stator blades with a stator blade having anintermediate portion which is curved to a shape similar to the shapeassumed by each of said straight stator blades while vibrating in itspredominate mode of vibration, but which is otherwise substantiallyidentical to said intermediate portion of a corresponding said straightstator blade.

2. A method as claimed in claim 1 including the step of replacing eachof the straight stator blades with a stator blade having an intermediateportion which is curved to a shape similar to the shape which would beassumed by a straight, but otherwise substantially identical blade inuse when its predominate mode of vibrationis its first flap mode.

3. A method as claimed in claim 1 including the step of replacing eachof the straight stator blades with a stator blade having an intermediateportion which is curved to a shape similar to the shape which would beassumed by a straight, but otherwise substantially identical blade inuse when its predominate mode of vibration is is second flap, mode.

4. A method as claimed in claim 1 including the step of replacing eachof the straight stator blades with a stator blade having an intermediateportion which is curved to a shape similar to the shape which would beassumed by a straight, but otherwise substantially identical blade inuse when its predominate mode of vibration is its third flap mode.

1. A method of manufacturing an assembly of stator blades having meanssecuring each blade at its radially inner and outer portion, said methodcomprising the following steps: a. producing an annular array of statorblades, each blade having an intermediate portion which is substantiallystraight in a radial sense, b. subjecting said array of stator blades tooperating conditions substantially identical to the desired operatingconditions of said stator blades, c. determining the predominate mode ofvibration of each of said stator blades under said operating conditions,and d. replacing each of said stator blades with a stator blade havingan intermediate portion which is curved to a shape similar to the shapeassumed by each of said straight stator blades while vibrating in itspredominate mode of vibration, but which is otherwise substantiallyidentical to said intermediate portion of a corresponding said straightstator blade.
 2. A method as claimed in claim 1 including the step ofreplacing each of the straight stator blades with a stator blade havingan intermediate portIon which is curved to a shape similar to the shapewhich would be assumed by a straight, but otherwise substantiallyidentical blade in use when its predominate mode of vibration is itsfirst flap mode.
 3. A method as claimed in claim 1 including the step ofreplacing each of the straight stator blades with a stator blade havingan intermediate portion which is curved to a shape similar to the shapewhich would be assumed by a straight, but otherwise substantiallyidentical blade in use when its predominate mode of vibration is issecond flap mode.
 4. A method as claimed in claim 1 including the stepof replacing each of the straight stator blades with a stator bladehaving an intermediate portion which is curved to a shape similar to theshape which would be assumed by a straight, but otherwise substantiallyidentical blade in use when its predominate mode of vibration is itsthird flap mode.